Signaling of reference picture resampling with constant window size indication in video bitstream

ABSTRACT

A method of decoding an encoded video bitstream using at least one processor, including obtaining a first flag indicating whether a constant picture size is used in a coded video sequence including a current picture; based on the first flag indicating that the constant picture size is used, decoding the current picture without performing reference picture resampling; based on the first flag indicating that the constant picture size is not used, obtaining a second flag indicating whether a conformance window size is signaled; based on the second flag indicating that the conformance window size is signaled: obtaining the conformance window size, determining a resampling ratio between the current picture and a reference picture based on the conformance window size, and performing the reference picture resampling on the current picture using the resampling ratio.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. § 119 to U.S.Provisional Application No. 62/903,601, filed on Sep. 20, 2019, in theUnited States Patent & Trademark Office, the disclosures of which areincorporated herein by reference in their entirety.

FIELD

The disclosed subject matter relates to video coding and decoding, andmore specifically, to the signaling of a size of a picture, or parts ofa picture, that may change from picture to picture or picture part topicture part.

BACKGROUND

Video coding and decoding using inter-picture prediction with motioncompensation has been known. Uncompressed digital video can consist of aseries of pictures, each picture having a spatial dimension of, forexample, 1920×1080 luminance samples and associated chrominance samples.The series of pictures can have a fixed or variable picture rate(informally also known as frame rate), of, for example 60 pictures persecond or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GByte of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reducing aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signal is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision contribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding, some of which will be introducedbelow.

Historically, video encoders and decoders tended to operate on a givenpicture size that was, in most cases, defined and stayed constant for acoded video sequence (CVS), Group of Pictures (GOP), or a similarmulti-picture timeframe. For example, in MPEG-2, system designs areknown to change the horizontal resolution (and, thereby, the picturesize) dependent on factors such as activity of the scene, but only at Ipictures, hence typically for a GOP. The resampling of referencepictures for use of different resolutions within a CVS is known, forexample, from ITU-T Rec. H.263 Annex P. However, here the picture sizedoes not change, only the reference pictures are being resampled,resulting potentially in only parts of the picture canvas being used (incase of downsampling), or only parts of the scene being captured (incase of upsampling). Further, H.263 Annex Q allows the resampling of anindividual macroblock by a factor of two (in each dimension), upward ordownward. Again, the picture size remains the same. The size of amacroblock is fixed in H.263, and therefore does not need to besignaled.

Changes of picture size in predicted pictures became more mainstream inmodern video coding. For example, VP9 allows reference pictureresampling and change of resolution for a whole picture. Similarly,certain proposals made towards VVC (including, for example, Hendry, et.al, “On adaptive resolution change (ARC) for VVC”, Joint Video Teamdocument JVET-M0135-v1, Jan. 9-19, 2019, incorporated herein in itsentirety) allow for resampling of whole reference pictures todifferent—higher or lower—resolutions. In that document, differentcandidate resolutions are suggested to be coded in the sequenceparameter set and referred to by per-picture syntax elements in thepicture parameter set.

SUMMARY

In an embodiment, there is provided a method of decoding an encodedvideo bitstream using at least one processor, the method includingobtaining a first flag indicating whether a constant picture size isused in a coded video sequence including a current picture; based on thefirst flag indicating that the constant picture size is used, decodingthe current picture without performing reference picture resampling;based on the first flag indicating that the constant picture size is notused, obtaining a second flag indicating whether a conformance windowsize is signaled; based on the second flag indicating that theconformance window size is signaled: obtaining the conformance windowsize, determining a resampling ratio between the current picture and areference picture based on the conformance window size, and performingthe reference picture resampling on the current picture using theresampling ratio.

In an embodiment, there is provided a device for decoding an encodedvideo bitstream, the device including at least one memory configured tostore program code; and at least one processor configured to read theprogram code and operate as instructed by the program code, the programcode including: first obtaining code configured to cause the at leastone processor to obtain a first flag indicating whether a constantpicture size is used in a coded video sequence including a currentpicture; decoding code configured to cause the at least one processorto, based on the first flag indicating that the constant picture size isused, decode the current picture without performing reference pictureresampling; second obtaining code configured to cause the at least oneprocessor to, based on the first flag indicating that the constantpicture size is not used, obtain a second flag indicating whether aconformance window size is signaled; and performing code configured tocause the at least one processor to, based on the second flag indicatingthat the conformance window size is signaled, obtain the conformancewindow size, determine a resampling ratio between the current pictureand a reference picture based on the conformance window size, andperform the reference picture resampling on the current picture usingthe resampling ratio.

In an embodiment, there is provided a non-transitory computer-readablemedium storing instructions, the instructions including: one or moreinstructions that, when executed by one or more processors of a devicefor decoding an encoded video bitstream, cause the one or moreprocessors to: obtain a first flag indicating whether a constant picturesize is used in a coded video sequence including a current picture;based on the first flag indicating that the constant picture size isused, decode the current picture without performing reference pictureresampling; based on the first flag indicating that the constant picturesize is not used, obtain a second flag indicating whether a conformancewindow size is signaled; based on the second flag indicating that theconformance window size is signaled: obtain the conformance window size,determine a resampling ratio between the current picture and a referencepicture based on the conformance window size, and perform the referencepicture resampling on the current picture using the resampling ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 5 is a schematic illustration of options for signaling ARC/RPRparameters in accordance with an embodiment.

FIGS. 6A-6B are schematic illustration of examples of syntax tables inaccordance with an embodiment.

FIG. 7 is a schematic illustration of signaling picture size andconformance window in SPS in accordance with an embodiment.

FIG. 8 is a schematic illustration of signaling picture size andconformance window in PPS in accordance with embodiments.

FIG. 9 is a flowchart of an example process for decoding an encodedvideo bitstream in accordance with an embodiment.

FIG. 10 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION

FIG. 1 illustrates a simplified block diagram of a communication system(100) according to an embodiment of the present disclosure. The system(100) may include at least two terminals (110-120) interconnected via anetwork (150). For unidirectional transmission of data, a first terminal(110) may code video data at a local location for transmission to theother terminal (120) via the network (150). The second terminal (120)may receive the coded video data of the other terminal from the network(150), decode the coded data and display the recovered video data.Unidirectional data transmission may be common in media servingapplications and the like.

FIG. 1 illustrates a second pair of terminals (130, 140) provided tosupport bidirectional transmission of coded video that may occur, forexample, during videoconferencing. For bidirectional transmission ofdata, each terminal (130, 140) may code video data captured at a locallocation for transmission to the other terminal via the network (150).Each terminal (130, 140) also may receive the coded video datatransmitted by the other terminal, may decode the coded data and maydisplay the recovered video data at a local display device.

In FIG. 1, the terminals (110-140) may be illustrated as servers,personal computers and smart phones but the principles of the presentdisclosure may be not so limited. Embodiments of the present disclosurefind application with laptop computers, tablet computers, media playersand/or dedicated video conferencing equipment. The network (150)represents any number of networks that convey coded video data among theterminals (110-140), including for example wireline and/or wirelesscommunication networks. The communication network (150) may exchangedata in circuit-switched and/or packet-switched channels. Representativenetworks include telecommunications networks, local area networks, widearea networks and/or the Internet. For the purposes of the presentdiscussion, the architecture and topology of the network (150) may beimmaterial to the operation of the present disclosure unless explainedherein below.

FIG. 2 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and decoder in astreaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (213), that caninclude a video source (201), for example a digital camera, creating afor example uncompressed video sample stream (202). That sample stream(202), depicted as a bold line to emphasize a high data volume whencompared to encoded video bitstreams, can be processed by an encoder(203) coupled to the camera (201). The encoder (203) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video bitstream (204), depicted as a thin line toemphasize the lower data volume when compared to the sample stream, canbe stored on a streaming server (205) for future use. One or morestreaming clients (206, 208) can access the streaming server (205) toretrieve copies (207, 209) of the encoded video bitstream (204). Aclient (206) can include a video decoder (210) which decodes theincoming copy of the encoded video bitstream (207) and creates anoutgoing video sample stream (211) that can be rendered on a display(212) or other rendering device (not depicted). In some streamingsystems, the video bitstreams (204, 207, 209) can be encoded accordingto certain video coding/compression standards. Examples of thosestandards include ITU-T Recommendation H.265. Under development is avideo coding standard informally known as Versatile Video Coding or VVC.The disclosed subject matter may be used in the context of VVC.

FIG. 3 may be a functional block diagram of a video decoder (210)according to an embodiment of the present disclosure.

A receiver (310) may receive one or more codec video sequences to bedecoded by the decoder (210); in the same or another embodiment, onecoded video sequence at a time, where the decoding of each coded videosequence is independent from other coded video sequences. The codedvideo sequence may be received from a channel (312), which may be ahardware/software link to a storage device which stores the encodedvideo data. The receiver (310) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (310) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (315) may be coupled inbetween receiver (310) and entropy decoder/parser (320) (“parser”henceforth). When receiver (310) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosychronous network, the buffer (315) may not be needed, or can besmall. For use on best effort packet networks such as the Internet, thebuffer (315) may be required, can be comparatively large and canadvantageously of adaptive size.

The video decoder (210) may include a parser (320) to reconstructsymbols (321) from the entropy coded video sequence. Categories of thosesymbols include information used to manage operation of the decoder(210), and potentially information to control a rendering device such asa display (212) that is not an integral part of the decoder but can becoupled to it, as was shown in FIG. 3. The control information for therendering device(s) may be in the form of Supplementary EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (320) mayparse/entropy-decode the coded video sequence received. The coding ofthe coded video sequence can be in accordance with a video codingtechnology or standard, and can follow principles well known to a personskilled in the art, including variable length coding, Huffman coding,arithmetic coding with or without context sensitivity, and so forth. Theparser (320) may extract from the coded video sequence, a set ofsubgroup parameters for at least one of the subgroups of pixels in thevideo decoder, based upon at least one parameter corresponding to thegroup. Subgroups can include Groups of Pictures (GOPs), pictures,sub-pictures, tiles, slices, bricks, macroblocks, Coding Tree Units(CTUs) Coding Units (CUs), blocks, Transform Units (TUs), PredictionUnits (PUs) and so forth. A tile may indicate a rectangular region ofCU/CTUs within a particular tile column and row in a picture. A brickmay indicate a rectangular region of CU/CTU rows within a particulartile. A slice may indicate one or more bricks of a picture, which arecontained in an NAL unit. A sub-picture may indicate an rectangularregion of one or more slices in a picture. The entropy decoder/parsermay also extract from the coded video sequence information such astransform coefficients, quantizer parameter values, motion vectors, andso forth.

The parser (320) may perform entropy decoding/parsing operation on thevideo sequence received from the buffer (315), so to create symbols(321).

Reconstruction of the symbols (321) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (320). The flow of such subgroup control information between theparser (320) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, decoder 210 can beconceptually subdivided into a number of functional units as describedbelow. In a practical implementation operating under commercialconstraints, many of these units interact closely with each other andcan, at least partly, be integrated into each other. However, for thepurpose of describing the disclosed subject matter, the conceptualsubdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (351). Thescaler/inverse transform unit (351) receives quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (321) from the parser (320). It can output blockscomprising sample values, that can be input into aggregator (355).

In some cases, the output samples of the scaler/inverse transform (351)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (352). In some cases, the intra pictureprediction unit (352) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current (partly reconstructed) picture(358). The aggregator (355), in some cases, adds, on a per sample basis,the prediction information the intra prediction unit (352) has generatedto the output sample information as provided by the scaler/inversetransform unit (351).

In other cases, the output samples of the scaler/inverse transform unit(351) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a Motion Compensation Prediction unit (353) canaccess reference picture memory (357) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (321) pertaining to the block, these samples can beadded by the aggregator (355) to the output of the scaler/inversetransform unit (in this case called the residual samples or residualsignal) so to generate output sample information. The addresses withinthe reference picture memory form where the motion compensation unitfetches prediction samples can be controlled by motion vectors,available to the motion compensation unit in the form of symbols (321)that can have, for example X, Y, and reference picture components.Motion compensation also can include interpolation of sample values asfetched from the reference picture memory when sub-sample exact motionvectors are in use, motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (355) can be subject to variousloop filtering techniques in the loop filter unit (356). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video bitstream andmade available to the loop filter unit (356) as symbols (321) from theparser (320), but can also be responsive to meta-information obtainedduring the decoding of previous (in decoding order) parts of the codedpicture or coded video sequence, as well as responsive to previouslyreconstructed and loop-filtered sample values.

The output of the loop filter unit (356) can be a sample stream that canbe output to the render device (212) as well as stored in the referencepicture memory for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. Once a coded picture is fullyreconstructed and the coded picture has been identified as a referencepicture (by, for example, parser (320)), the current reference picture(358) can become part of the reference picture buffer (357), and a freshcurrent picture memory can be reallocated before commencing thereconstruction of the following coded picture.

The video decoder 210 may perform decoding operations according to apredetermined video compression technology that may be documented in astandard, such as ITU-T Rec. H.265. The coded video sequence may conformto a syntax specified by the video compression technology or standardbeing used, in the sense that it adheres to the syntax of the videocompression technology or standard, as specified in the videocompression technology document or standard and specifically in theprofiles document therein. Also necessary for compliance can be that thecomplexity of the coded video sequence is within bounds as defined bythe level of the video compression technology or standard. In somecases, levels restrict the maximum picture size, maximum frame rate,maximum reconstruction sample rate (measured in, for example megasamplesper second), maximum reference picture size, and so on. Limits set bylevels can, in some cases, be further restricted through HypotheticalReference Decoder (HRD) specifications and metadata for HRD buffermanagement signaled in the coded video sequence.

In an embodiment, the receiver (310) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (210) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or SNR enhancementlayers, redundant slices, redundant pictures, forward error correctioncodes, and so on.

FIG. 4 may be a functional block diagram of a video encoder (203)according to an embodiment of the present disclosure.

The encoder (203) may receive video samples from a video source (201)(that is not part of the encoder) that may capture video image(s) to becoded by the encoder (203).

The video source (201) may provide the source video sequence to be codedby the encoder (203) in the form of a digital video sample stream thatcan be of any suitable bit depth (for example: 8 bit, 10 bit, 12 bit, .. . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ) and anysuitable sampling structure (for example Y CrCb 4:2:0, Y CrCb 4:4:4). Ina media serving system, the video source (201) may be a storage devicestoring previously prepared video. In a videoconferencing system, thevideo source (203) may be a camera that captures local image informationas a video sequence. Video data may be provided as a plurality ofindividual pictures that impart motion when viewed in sequence. Thepictures themselves may be organized as a spatial array of pixels,wherein each pixel can comprise one or more sample depending on thesampling structure, color space, etc. in use. A person skilled in theart can readily understand the relationship between pixels and samples.The description below focusses on samples.

According to an embodiment, the encoder (203) may code and compress thepictures of the source video sequence into a coded video sequence (443)in real time or under any other time constraints as required by theapplication. Enforcing appropriate coding speed is one function ofController (450). Controller controls other functional units asdescribed below and is functionally coupled to these units. The couplingis not depicted for clarity. Parameters set by controller can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. A person skilled in the art can readily identify other functionsof controller (450) as they may pertain to video encoder (203) optimizedfor a certain system design.

Some video encoders operate in what a person skilled in the are readilyrecognizes as a “coding loop”. As an oversimplified description, acoding loop can consist of the encoding part of an encoder (430)(“source coder” henceforth) (responsible for creating symbols based onan input picture to be coded, and a reference picture(s)), and a (local)decoder (433) embedded in the encoder (203) that reconstructs thesymbols to create the sample data a (remote) decoder also would create(as any compression between symbols and coded video bitstream islossless in the video compression technologies considered in thedisclosed subject matter). That reconstructed sample stream is input tothe reference picture memory (434). As the decoding of a symbol streamleads to bit-exact results independent of decoder location (local orremote), the reference picture buffer content is also bit exact betweenlocal encoder and remote encoder. In other words, the prediction part ofan encoder “sees” as reference picture samples exactly the same samplevalues as a decoder would “see” when using prediction during decoding.This fundamental principle of reference picture synchronicity (andresulting drift, if synchronicity cannot be maintained, for examplebecause of channel errors) is well known to a person skilled in the art.

The operation of the “local” decoder (433) can be the same as of a“remote” decoder (210), which has already been described in detail abovein conjunction with FIG. 3. Briefly referring also to FIG. 4, however,as symbols are available and en/decoding of symbols to a coded videosequence by entropy coder (445) and parser (320) can be lossless, theentropy decoding parts of decoder (210), including channel (312),receiver (310), buffer (315), and parser (320) may not be fullyimplemented in local decoder (433).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focusses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

As part of its operation, the source coder (430) may perform motioncompensated predictive coding, which codes an input frame predictivelywith reference to one or more previously-coded frames from the videosequence that were designated as “reference frames.” In this manner, thecoding engine (432) codes differences between pixel blocks of an inputframe and pixel blocks of reference frame(s) that may be selected asprediction reference(s) to the input frame.

The local video decoder (433) may decode coded video data of frames thatmay be designated as reference frames, based on symbols created by thesource coder (430). Operations of the coding engine (432) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 4), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (433) replicates decodingprocesses that may be performed by the video decoder on reference framesand may cause reconstructed reference frames to be stored in thereference picture cache (434). In this manner, the encoder (203) maystore copies of reconstructed reference frames locally that have commoncontent as the reconstructed reference frames that will be obtained by afar-end video decoder (absent transmission errors).

The predictor (435) may perform prediction searches for the codingengine (432). That is, for a new frame to be coded, the predictor (435)may search the reference picture memory (434) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(435) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (435), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (434).

The controller (450) may manage coding operations of the video coder(430), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (445). The entropy coder translatesthe symbols as generated by the various functional units into a codedvideo sequence, by loss-less compressing the symbols according totechnologies known to a person skilled in the art as, for exampleHuffman coding, variable length coding, arithmetic coding, and so forth.

The transmitter (440) may buffer the coded video sequence(s) as createdby the entropy coder (445) to prepare it for transmission via acommunication channel (460), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(440) may merge coded video data from the video coder (430) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (450) may manage operation of the encoder (203). Duringcoding, the controller (450) may assign to each coded picture a certaincoded picture type, which may affect the coding techniques that may beapplied to the respective picture. For example, pictures often may beassigned as one of the following frame types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other frame in the sequence as a source of prediction.Some video codecs allow for different types of Intra pictures,including, for example Independent Decoder Refresh Pictures. A personskilled in the art is aware of those variants of I pictures and theirrespective applications and features.

A Predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A Bi-directionally Predictive Picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded non-predictively,via spatial prediction or via temporal prediction with reference to onepreviously coded reference pictures. Blocks of B pictures may be codednon-predictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video coder (203) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video coder (203) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (440) may transmit additional datawith the encoded video. The video coder (430) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, Supplementary EnhancementInformation (SEI) messages, Visual Usability Information (VUI) parameterset fragments, and so on.

Recently, compressed domain aggregation or extraction of multiplesemantically independent picture parts into a single video picture hasgained some attention. In particular, in the context of, for example,360 coding or certain surveillance applications, multiple semanticallyindependent source pictures (for examples the six cube surface of acube-projected 360 scene, or individual camera inputs in case of amulti-camera surveillance setup) may require separate adaptiveresolution settings to cope with different per-scene activity at a givenpoint in time. In other words, encoders, at a given point in time, maychoose to use different resampling factors for different semanticallyindependent pictures that make up the whole 360 or surveillance scene.When combined into a single picture, that, in turn, requires thatreference picture resampling is performed, and adaptive resolutioncoding signaling is available, for parts of a coded picture.

Below, a few terms will be introduced that will be referred to in theremainder of this description.

Sub-Picture may refer to a, in some cases, rectangular arrangement ofsamples, blocks, macroblocks, coding units, or similar entities that aresemantically grouped, and that may be independently coded in changedresolution. One or more sub-pictures may form a picture. One or morecoded sub-pictures may form a coded picture. One or more sub-picturesmay be assembled into a picture, and one or more sub pictures may beextracted from a picture. In certain environments, one or more codedsub-pictures may be assembled in the compressed domain withouttranscoding to the sample level into a coded picture, and in the same orother cases, one or more coded sub-pictures may be extracted from acoded picture in the compressed domain.

Reference Picture Resampling (RPR) or Adaptive Resolution Change (ARC)may refer to mechanisms that allow the change of resolution of a pictureor sub-picture within a coded video sequence, by the means of, forexample, reference picture resampling. RPR/ARC parameters henceforthrefer to the control information required to perform adaptive resolutionchange, that may include, for example, filter parameters, scalingfactors, resolutions of output and/or reference pictures, variouscontrol flags, and so forth.

In embodiments coding and decoding may be performed on a single,semantically independent coded video picture. Before describing theimplication of coding/decoding of multiple sub pictures with independentRPR/ARC parameters and its implied additional complexity, options forsignaling RPR/ARC parameters shall be described.

Referring to FIG. 5, shown are several embodiments for signaling RPR/ARCparameters. As noted with each of the embodiments, they may have certainadvantages and certain disadvantages from a coding efficiency,complexity, and architecture viewpoint. A video coding standard ortechnology may choose one or more of these embodiments, or options knownfrom related art, for signaling RPR/ARC parameters. The embodiments maynot be mutually exclusive, and conceivably may be interchanged based onapplication needs, standards technology involved, or encoder's choice.

Classes of RPR/ARC parameters may include:

-up/downsample factors, separate or combined in X and Y dimension

-up/downsample factors, with an addition of a temporal dimension,indicating constant speed zoom in/out for a given number of pictures

-Either of the above two may involve the coding of one or morepresumably short syntax elements that may point into a table containingthe factor(s).

-resolution, in X or Y dimension, in units of samples, blocks,macroblocks, coding units (CUs), or any other suitable granularity, ofthe input picture, output picture, reference picture, coded picture,combined or separately. If there is more than one resolution (such as,for example, one for input picture, one for reference picture) then, incertain cases, one set of values may be inferred to from another set ofvalues. Such could be gated, for example, by the use of flags. For amore detailed example, see below.

“warping” coordinates akin those used in H.263 Annex P, again in asuitable granularity as described above. H.263 Annex P defines oneefficient way to code such warping coordinates, but other, potentiallymore efficient ways could conceivably also be devised. For example, thevariable length reversible, “Huffman”-style coding of warpingcoordinates of Annex P could be replaced by a suitable length binarycoding, where the length of the binary code word could, for example, bederived from a maximum picture size, possibly multiplied by a certainfactor and offset by a certain value, so to allow for “warping” outsideof the maximum picture size's boundaries.

-up or downsample filter parameters. In embodiments, there may be only asingle filter for up and/or downsampling. However, in embodiments, itcan be desirable to allow more flexibility in filter design, and thatmay require to signaling of filter parameters. Such parameters may beselected through an index in a list of possible filter designs, thefilter may be fully specified (for example through a list of filtercoefficients, using suitable entropy coding techniques), the filter maybe implicitly selected through up/downsample ratios according which inturn are signaled according to any of the mechanisms mentioned above,and so forth.

Henceforth, the description assumes the coding of a finite set ofup/downsample factors (the same factor to be used in both X and Ydimension), indicated through a codeword. That codeword may be variablelength coded, for example using the Ext-Golomb code common for certainsyntax elements in video coding specifications such as H.264 and H.265.One suitable mapping of values to up/downsample factors can, forexample, be according to Table 1:

TABLE 1 Ext-Golomb Original/ Codeword Code Target resolution 0 1 1/1 1010 1/1.5 (upscale by 50%) 2 011 1.5/1 (downscale by 50%) 3 00100 1/2(upscale by 100%) 4 00101 2/1 (downscale by 100%)

Many similar mappings could be devised according to the needs of anapplication and the capabilities of the up and downscale mechanismsavailable in a video compression technology or standard. The table couldbe extended to more values. Values may also be represented by entropycoding mechanisms other than Ext-Golomb codes, for example using binarycoding. That may have certain advantages when the resampling factorswere of interest outside the video processing engines (encoder anddecoder foremost) themselves, for example by MANES. It should be notedthat, for situations where no resolution change is required, anExt-Golomb code can be chosen that is short; in the table above, only asingle bit. That can have a coding efficiency advantage over usingbinary codes for the most common case.

The number of entries in the table, as well as their semantics, may befully or partially configurable. For example, the basic outline of thetable may be conveyed in a “high” parameter set such as a sequence ordecoder parameter set. In embodiments, one or more such tables may bedefined in a video coding technology or standard, and may be selectedthrough for example a decoder or sequence parameter set.

Below is described how an upsample/downsample factor (ARC information),coded as described above, may be included in a video coding technologyor standard syntax. Similar considerations may apply to one, or a few,codewords controlling up/downsample filters. See below for a discussionwhen comparatively large amounts of data are required for a filter orother data structures.

As shown in FIG. 5, H.263 Annex P includes the ARC information (502) inthe form of four warping coordinates into the picture header (501),specifically in the H.263 PLUSPTYPE (503) header extension. This can bea sensible design choice when a) there is a picture header available,and b) frequent changes of the ARC information are expected. However,the overhead when using H.263-style signaling can be quite high, andscaling factors may not pertain among picture boundaries as pictureheader can be of transient nature.

In the same or another embodiment, the signaling of ARC parameters canfollow a detailed example as outlined in FIGS. 6A-6B. FIGS. 6A-6B depictsyntax diagrams in a type of representation using a notation whichroughly follows C-style programming, as for example used in video codingstandards since at least 1993. Lines in boldface indicate syntaxelements present in the bitstream, lines without boldface often indicatecontrol flow or the setting of variables.

As shown in FIG. 6A, a tile group header (601) as an exemplary syntaxstructure of a header applicable to a (possibly rectangular) part of apicture can conditionally contain, a variable length, Exp-Golomb codedsyntax element decpic size_idx (602) (depicted in boldface). Thepresence of this syntax element in the tile group header can be gated onthe use of adaptive resolution (603)—here, the value of a flag notdepicted in boldface, which means that flag is present in the bitstreamat the point where it occurs in the syntax diagram. Whether or notadaptive resolution is in use for this picture or parts thereof can besignaled in any high level syntax structure inside or outside thebitstream. In the example shown, it is signaled in the sequenceparameter set as outlined below.

Referring to FIG. 6B, shown is also an excerpt of a sequence parameterset (610). The first syntax element shown is adaptivepic resolutionchange flag (611). When true, that flag can indicate the use of adaptiveresolution which, in turn may require certain control information. Inthe example, such control information is conditionally present based onthe value of the flag based on the if( )statement in the parameter set(612) and the tile group header (601).

When adaptive resolution is in use, in this example, coded is an outputresolution in units of samples (613). The numeral 613 refers to bothoutput_pic_width_in_luma_samples and output_pic_height_in_luma_samples,which together can define the resolution of the output picture.Elsewhere in a video coding technology or standard, certain restrictionsto either value can be defined. For example, a level definition maylimit the number of total output samples, which could be the product ofthe value of those two syntax elements. Also, certain video codingtechnologies or standards, or external technologies or standards suchas, for example, system standards, may limit the numbering range (forexample, one or both dimensions must be divisible by a power of 2number), or the aspect ratio (for example, the width and height must bein a relation such as 4:3 or 16:9). Such restrictions may be introducedto facilitate hardware implementations or for other reasons, and arewell known in the art.

In certain applications, it can be advisable that the encoder instructsthe decoder to use a certain reference picture size rather thanimplicitly assume that size to be the output picture size. In thisexample, the syntax element reference_pic_size_present_flag (614) gatesthe conditional presence of reference picture dimensions (615) (again,the numeral refers to both width and height).

Finally, shown is a table of possible decoding picture width andheights. Such a table can be expressed, for example, by a tableindication (num_decpic_size in_luma_samples_minus1) (616). The “minus1”can refer to the interpretation of the value of that syntax element. Forexample, if the coded value is zero, one table entry is present. If thevalue is five, six table entries are present. For each “line” in thetable, decoded picture width and height are then included in the syntax(617).

The table entries presented (617) can be indexed using the syntaxelement decpic size idx (602) in the tile group header, thereby allowingdifferent decoded sizes—in effect, zoom factors—per tile group.

Certain video coding technologies or standards, for example VP9, supportspatial scalability by implementing certain forms of reference pictureresampling (signaled quite differently from the disclosed subjectmatter) in conjunction with temporal scalability, so to enable spatialscalability. In particular, certain reference pictures may be upsampledusing ARC-style technologies to a higher resolution to form the base ofa spatial enhancement layer. Those upsampled pictures could be refined,using normal prediction mechanisms at the high resolution, so to adddetail.

Embodiments discussed herein can be used in such an environment. Incertain cases, in the same or another embodiment, a value in the NALunit header, for example the Temporal ID field, can be used to indicatenot only the temporal but also the spatial layer. Doing so may havecertain advantages for certain system designs; for example, existingSelected Forwarding Units (SFU) created and optimized for temporal layerselected forwarding based on the NAL unit header Temporal ID value canbe used without modification, for scalable environments. In order toenable that, there may be a requirement for a mapping between the codedpicture size and the temporal layer is indicated by the temporal IDfield in the NAL unit header.

Recently, compressed domain aggregation or extraction of multiplesemantically independent picture parts into a single video picture hasgained some attention. In particular, in the context of, for example,360 coding or certain surveillance applications, multiple semanticallyindependent source pictures (for examples the six cube surface of acube-projected 360 scene, or individual camera inputs in case of amulti-camera surveillance setup) may require separate adaptiveresolution settings to cope with different per-scene activity at a givenpoint in time. In other words, encoders, at a given point in time, maychoose to use different resampling factors for different semanticallyindependent pictures that make up the whole 360 or surveillance scene.When combined into a single picture, that, in turn, requires thatreference picture resampling is performed, and adaptive resolutioncoding signaling is available, for parts of a coded picture.

In embodiments, not all samples of a reconstructed picture are intendedfor output. An encoder can indicate a rectangular sub-part of picture asintended for output using a conformance window. The conformance windowmay be described or indicated through, for example, a left and rightoffset from the picture edges as defined by the picture size. Certainuse cases can be identified where conformance windows may be relevant,including overscan, spatial assembly of views in Multiview systems, or360 systems where a conformance windows may indicate one of several cubemap surfaces to be output.

Because not all application require the use of a conformance window andbecause the conformance window parameters may require a certain amountof bits in the bitstream and hence, when not used, may harm codingefficiency, the presence of such parameters may be gated by a flag.

In embodiments, a conformance window size may be signaled in a pictureparameter set (PPS). Conformance window parameters which may specify theconformance window size may be used for calculating the resamplingratio, when the conformance window size of the reference picture isdifferent from that of the current picture. A decoder may need torecognize the conformance window size of each picture, to determinewhether the resampling process is needed. When the resampling ratio isnot equal to 1, the output picture size is not constant within a CVS,and special handling and post processing of output pictures like anup-/down-scaling for display may be used.

In embodiments, a flag that indicates whether the decoded/output picturehas the same size and the resampling ratio is equal to 1 within aCVS/bitstream, may be signaled in a high-level parameter set such as adecoding parameter set (DPS), video parameter set (VPS), or sequenceparameter set (SPS). The flag may be used for session negotiation forvideo streaming or configuration of decoder and display setting.

Referring to FIG. 7, a flag constantpic size flag (704) equal to 1 mayindicate that the picture sizes of the pictures in the CVS are the same.constantpic size flag equal to 0 may indicate that the picture sizes ofthe pictures in the CVS may or may not be the same. If the value ofconstantpic size flag is equal to 1, a flag sps_conformance_window_flag(705) may be present in SPS (701). sps_conformance_window_flag equal to1 may indicate that the conformance cropping window offset parametersfollow at a suitable location, for example, next, in the SPS.sps_conformance_window_flag equal to 0 may indicate that the conformancecropping window offset parameters are not present.

In embodiments, sps_conf_win_left_offset (706),sps_conf_win_right_offset (707), sps_conf_win_top_offset (708), andsps_conf_win_bottom_offset (709) may specify the samples of the picturesin the CVS that are output from the decoding process, in terms of arectangular region specified in picture coordinates for output.

In embodiments, when the syntax elements sps_conf_win_left_offset,sps_conf_win_right_offset, sps_conf_win_top_offset, andsps_conf_win_bottom_offset are not present, the values ofsps_conf_win_left_offset, sps_conf_win_right_offset,sps_conf_win_top_offset, and sps_conf_win_bottom_offset may be inferredto be equal to 0.

In embodiments, referring to FIG. 8, pic_width_in_luma_samples (802) mayspecifiy the width of each decoded picture referring to the PPS (801) inunits of luma samples. In embodiments, pic_width_in_luma_samples may benot equal to 0, may be an integer multiple of Max(8, MinCbSizeY), andmay be less than or equal to pic_width_max in_luma_samples. When notpresent, the value of pic_width_in_luma_samples may be inferred to beequal to pic_width_max_in_luma_samples. pic_height_in_luma_samples (803)may specify the height of each decoded picture referring to the PPS inunits of luma samples. pic_height_in_luma_samples may, in some cases, benot equal to 0 and may be an integer multiple of Max(8, MinCbSizeY), andmay be less than or equal to pic_height_max_in_luma_samples. When notpresent, the value of pic_height_in_luma_samples may be inferred to beequal to pic_height_max_in_luma_samples.

In embodiments, still referring to FIG. 8, conformance window_flag (804)equal to 1 may indicate that the conformance cropping window offsetparameters follow at a suitable location, for example next in the PPS(801). conformance window_flag equal to 0 may indicate that theconformance cropping window offset parameters are not present.conf_win_left_offset (805), conf_win_right_offset (806),conf_win_top_offset (807), and conf_win_bottom_offset (808) may specifythe samples of the pictures referring to the PPS that are output fromthe decoding process, in terms of a rectangular region specified inpicture coordinates for output.

In the same embodiment, when the syntax elements conf_win_left_offset,conf_win_right_offset, conf_win_top_offset, and conf_win_bottom_offsetare not present, the values of conf_win_left_offset,conf_win_right_offset, conf_win_top_offset, and conf_win_bottom_offsetmay be inferred to be equal to the values of sps_conf_win_left_offset,sps_conf_win_right_offset, sps_conf_win_top_offset, andsps_conf_win_bottom_offset, respectively.

In embodiments, the conformance cropping window may contain the lumasamples with horizontal picture coordinates fromSubWidthC*conf_win_left_offset topic_width_in_luma_samples−(SubWidthC*conf_win_right_offset+1) andvertical picture coordinates from SubHeightC*conf_win_top_offset to picheight_in luma samples−(SubHeightC*conf_win_bottom_offset+1), inclusive.

The value of SubWidthC*(conf_win_left_offset+conf_win_right_offset) maybe less than pic_width_in_luma_samples, and the value ofSubHeightC*(conf_win_top_offset+conf_win_bottom_offset) may be less thanpic_height_in_luma_samples.

The variables PicOutputWidthL and PicOutputHeightL may be derived asshown in Equation 1 and Equation 2 below:

PicOutputWidthL=pic_width_in_luma_samples−SubWidthC*(conf_win_right_offset+conf_win_left_offset)   (Equation 1)

PicOutputHeightL=pic_height_in_luma_samples−SubHeightC*(conf_win_bottom_offset+conf_win_top_offset)   (Equation 2)

In embodiments, the fractional interpolation process with the referencepicture resampling may be processed as follows.

Inputs to this process may be a luma location (xSb, ySb) specifying thetop-left sample of the current coding subblock relative to the top leftluma sample of the current picture, avariable sbWidth specifying thewidth of the current coding subblock, a variable sbHeight specifying theheight of the current coding subblock, a motion vector offset mvOffset,a refined motion vector refMvLX, the selected reference picture samplearray refPicLX, the half sample interpolation filter index hpelIfldx,the bi-directional optical flow flag bdofFlag, and a variable cldxspecifying the colour component index of the current block.

Outputs of this process may be: an(sbWidth+brdExtSize)x(sbHeight+brdExtSize) array predSamplesLX ofprediction sample values.

The prediction block border extension size brdExtSize may be derived asshown in Equation 3 below:

brdExtSize=(bdofFlag | |(inter_affine_flag[xSb] [ ySb]&&sps_affine_prof_enabled_flag))?2:0   (Equation 3)

The variable fRefWidth may be set equal to the PicOutputWidthL of thereference picture in luma samples. The variable fRefHeight may be setequal to PicOutputHeightL of the reference picture in luma samples. Themotion vector mvLX may be set equal to (refMvLX−mvOffset).

If cIdx is equal to 0, the following may apply:

-   -   The scaling factors and their fixed-point representations may be        defined according to Equation 4 and Equation 5 below:

hori_scale_fp=((fRefWidth<<14)+(PicOutputWidthL>>1))/PicOutputWidthL  (Equation 4)

vert_scale_fp=((fRefHeight<<14)+(PicOutputHeightL>>1))/PicOutputHeightL  (Equation 5)

-   -   Let (xInt_(L), yInt_(L)) may be a luma location given in        full-sample units and (xFracL, yFracL) be an offset given in        1/16-sample units. These variables may be used in this clause        for specifying fractional-sample locations inside the reference        sample arrays refPicLX.    -   The top-left coordinate of the bounding block for reference        sample padding (xSbInt_(L), ySbInt_(L)) may be set equal to        (xSb+_(mvLX[0]>>4), ySb+_(mvLX[1]>>4)).    -   For each luma sample location (x_(L)=0..sbWidth−1+brdExtSize,        y_(L)=0..sbHeight−1+brdExtSize) inside the prediction luma        sample array predSamplesLX, the corresponding prediction luma        sample value predSamplesLX[x_(L)] [yL] is derived as follows:    -   Let (refxSb_(L), refySb_(L)) and (refx_(L), refy_(L)) be luma        locations pointed to by a motion vector (refMvLX, refMvLX) given        in 1/16-sample units. The variables refxSb_(L), refx_(L),        refySb_(L), and refyL may be derived as shown in Equations 6-9        below:

refxSb_(L)=((xSb<<4)+refMvLX[0])*hori_scale_fp   (Equation 6)

refx_(L)=((Sign(refxSb)*((Abs(refxSb)+128)>>8)+x_(L)*((hori_scale_fp+8)>>4))+32)>>6  (Equation 7)

refySbL=((ySb<<4)+refMvLX[1])*vert_scale_fp   (Equation 8)

refyL=((Sign(refySb)*((Abs(refySb)+128)>>8)+yL*((vert_scale_fp+8)>>4))+32)>>6   (Equation 9)

-   -   The variables xInt_(L), yInt_(L), xFrac_(L) and yFrac_(L) may be        derived as shown in Equations 10-13 below:

xInt_(L)=refx_(L)>>4   (Equation 10)

yInt_(L)=refy_(L)>>4   (Equation 11)

xFrac_(L)=refx_(L) & 15   (Equation 12)

yFrac_(L)=refy_(L) & 15   (Equation 13)

-   -   If bdofFlag is equal to TRUE or (sps_affine_prof_enabled_flag is        equal to TRUE and inter_affine_flag[xSb] [ySb] is equal to        TRUE), and one or more of the following conditions are true, the        prediction luma sample value predSamplesLX[ x_(L)] [y_(L)] may        be derived by invoking the luma integer sample fetching process        as specified in an appropriate clause of a video coding        specification, with (xInt_(L)+(xFrac_(L)>>3)−1),        yInt_(L)+(yFrac_(L)>>3)−1) and refPicLX as inputs.

1. x_(L) is equal to 0.

2. x_(L) is equal to sbWidth+1.

3. y_(L) is equal to 0.

4. y_(L) is equal to sbHeight+1.

-   -   Otherwise, the prediction luma sample value predSamplesLX[xL]        [yL ] may be derived by invoking the luma sample 8-tap        interpolation filtering process as specified in an appropriate        clause of a video coding specification with        (xIntL−(brdExtSize>0?1:0), yIntL−(brdExtSize>0?1:0)), (xFracL,        yFracL), (xSbInt_(L), ySbInt_(L)), refPicLX, hpelIfIdx, sbWidth,        sbHeight and (xSb, ySb) as inputs.

Otherwise (cIdx is not equal to 0), the following may apply:

-   -   1. Let (xIntC, yIntC) be a chroma location given in full-sample        units and (xFracC, yFracC) be an offset given in 1/32 sample        units. These variables may be used in this clause for specifying        general fractional-sample locations inside the reference sample        arrays refPicLX.    -   2. The top-left coordinate of the bounding block for reference        sample padding (xSbIntC, ySbIntC) is set equal to        ((xSb/SubWidthC)+(mvLX[0]>>5), (ySb/SubHeightC)+(mvLX[1]>>5)).    -   3. For each chroma sample location (xC=0..sbWidth−1,        yC=0..sbHeight−1) inside the prediction chroma sample arrays        predSamplesLX, the corresponding prediction chroma sample value        predSamplesLX[xC ] [yC ] may be derived as follows:    -   Let (refxSb_(C), refySb_(C)) and (refx_(C), refy_(C)) be chroma        locations pointed to by a motion vector (mvLX[0], mvLX[1]) given        in 1/32-sample units. The variables refxSb_(C), refySb_(C),        refx_(C) and refyc may be derived as shown in Equations 14-17        below:

refxSb_(C)=((xSb/SubWidthC<<5)+mvLX[0])*hori_scale_fp   (Equation 14)

refx_(C)=((Sign(refxSb_(C))*((Abs(refxSb_(C))+256)>>9)+xC*((hori_scale_fp+8)>>4))+16)>>5  (Equation 15)

refySb_(C)=((ySb/SubHeightC<<5)+mvLX[1])*vert_scale_fp   (Equation 16)

refy_(C)=((Sign(refySb_(C))*((Abs(refySb_(C))+256)>>9)+yC*((vert_scale_fp+8)>>4))+16)>>5  (Equation 17)

-   -   The variables xInt_(C), yInt_(C), xFrac_(C) and yFrac_(C) may be        derived as shown in Equations 18-21 below:

xInt_(C)=refx_(C)>>5   (Equation 18)

yInt_(C)=refy_(C)>>5   (Equation 19)

xFrac_(C)=refy_(C)& 31   (Equation 20)

yFrac_(C)=refy_(C)& 31   (Equation 21)

The prediction sample value predSamplesLX[ xC ][ yC ] may be derived byinvoking the process specified above with (xIntC, yIntC), (xFracC,yFracC), (xSbIntC, ySbIntC), sbWidth, sbHeight and refPicLX as inputs.

FIG. 9 is a flowchart is an example process 900 for decoding an encodedvideo bitstream. In some implementations, one or more process blocks ofFIG. 9 may be performed by decoder 210. In some implementations, one ormore process blocks of FIG. 9 may be performed by another device or agroup of devices separate from or including decoder 210, such as encoder203.

As shown in FIG. 9, process 900 may include obtaining a first flagindicating whether a constant picture size is used in a coded videosequence including a current picture (block 910).

As further shown in FIG. 9, process 900 may include, determining fromthe first flag whether the constant picture size is used (block 920).

As further shown in FIG. 9, process 900 may include, based on the firstflag indicating that the constant picture size is used (YES at block920), decoding the current picture without performing reference pictureresampling (block 930).

As further shown in FIG. 9, based on the first flag indicating that theconstant picture size is not used (NO at block 920), process 900 mayproceed to block 940, block 950, block 960, and block 970.

As further shown in FIG. 9, process 900 may include obtaining a secondflag indicating whether a conformance window size is signaled (block940).

As further shown in FIG. 9, process 900 may include, based on the secondflag indicating that the conformance window size is signaled, obtainingthe conformance window size (block 950), determining a resampling ratiobetween the current picture and a reference picture based on theconformance window size (block 960), and performing the referencepicture resampling on the current picture using the resampling ratio(block 970).

In an embodiment, the conformance window size may be signaled as atleast one offset distance from a border of the current picture.

In an embodiment, the first flag may be signaled in a sequence parameterset (SPS), and the second flag may be signaled in one from among the SPSand a picture parameter set (PPS).

In an embodiment, the second flag may be signaled in the SPS, and mayindicate whether SPS conformance window parameters are signaled in theSPS.

In an embodiment, based on the second flag indicating that the SPSconformance window parameters are signaled in the SPS, the conformancewindow size may be obtained based on the SPS conformance windowparameters.

In an embodiment, based on the first flag indicating that the picturesize is not constant, process 900 may include obtaining a third flagindicating whether PPS conformance window parameters are signaled in thePPS.

In an embodiment, based on the second flag indicating that the SPSconformance window parameters are signaled in the SPS, and the thirdflag indicating that the PPS conformance window parameters are notsignaled in the PPS, the conformance window size may be obtained basedon the SPS conformance window parameters.

In an embodiment, based on the second flag indicating that the SPSconformance window parameters are not signaled in the SPS, and the thirdflag indicating that the PPS conformance window parameters are signaledin the PPS, the conformance window size may be obtained based on the PPSconformance window parameters.

Although FIG. 9 shows example blocks of process 900, in someimplementations, process 900 may include additional blocks, fewerblocks, different blocks, or differently arranged blocks than thosedepicted in FIG. 9. Additionally, or alternatively, two or more of theblocks of process 900 may be performed in parallel.

Further, the proposed methods may be implemented by processing circuitry(e.g., one or more processors or one or more integrated circuits). Inone example, the one or more processors execute a program that is storedin a non-transitory computer-readable medium to perform one or more ofthe proposed methods.

The techniques described above can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media.

For example, FIG. 10 shows a computer system 1000 suitable forimplementing certain embodiments of the disclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by computer central processing units (CPUs),Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 10 for computer system 1000 are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system 1000.

Computer system 1000 may include certain human interface input devices.Such a human interface input device may be responsive to input by one ormore human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard 1001, mouse 1002, trackpad 1003, touch screen1010 and associated graphics adapter 1050, data-glove, joystick 1005,microphone 1006, scanner 1007, camera 1008.

Computer system 1000 may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen 1010, data-glove, or joystick 1005, but there can also betactile feedback devices that do not serve as input devices), audiooutput devices (such as: speakers 1009, headphones (not depicted)),visual output devices (such as screens 1010 to include cathode ray tube(CRT) screens, liquid-crystal display (LCD) screens, plasma screens,organic light-emitting diode (OLED) screens, each with or withouttouch-screen input capability, each with or without tactile feedbackcapability—some of which may be capable to output two dimensional visualoutput or more than three dimensional output through means such asstereographic output; virtual-reality glasses (not depicted),holographic displays and smoke tanks (not depicted)), and printers (notdepicted).

Computer system 1000 can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW1020 with CD/DVD or the like media 1021, thumb-drive 1022, removablehard drive or solid state drive 1023, legacy magnetic media such as tapeand floppy disc (not depicted), specialized ROM/ASIC/PLD based devicessuch as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system 1000 can also include interface(s) to one or morecommunication networks (1155). Networks can for example be wireless,wireline, optical. Networks can further be local, wide-area,metropolitan, vehicular and industrial, real-time, delay-tolerant, andso on. Examples of networks include local area networks such asEthernet, wireless LANs, cellular networks to include global systems formobile communications (GSM), third generation (3G), fourth generation(4G), fifth generation (5G), Long-Term Evolution (LTE), and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters (1154) that attached to certain generalpurpose data ports or peripheral buses (1149) (such as, for exampleuniversal serial bus (USB) ports of the computer system 1000; others arecommonly integrated into the core of the computer system 1000 byattachment to a system bus as described below (for example Ethernetinterface into a PC computer system or cellular network interface into asmartphone computer system). As an example, network 1055 may beconnected to peripheral bus 1049 using network interface 1054. Using anyof these networks, computer system 1000 can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces (1154) as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core 1040 of thecomputer system 1000.

The core 1040 can include one or more Central Processing Units (CPU)1041, Graphics Processing Units (GPU) 1042, specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)1043, hardware accelerators 1044 for certain tasks, and so forth. Thesedevices, along with Read-only memory (ROM) 1045, Random-access memory(RAM) 1046, internal mass storage such as internal non-user accessiblehard drives, solid-state drives (SSDs), and the like 1047, may beconnected through a system bus 1048. In some computer systems, thesystem bus 1048 can be accessible in the form of one or more physicalplugs to enable extensions by additional CPUs, GPU, and the like. Theperipheral devices can be attached either directly to the core's systembus 1048, or through a peripheral bus 1049. Architectures for aperipheral bus include peripheral component interconnect (PCI), USB, andthe like.

CPUs 1041, GPUs 1042, FPGAs 1043, and accelerators 1044 can executecertain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM1045 or RAM 1046. Transitional data can be also be stored in RAM 1046,whereas permanent data can be stored for example, in the internal massstorage 1047. Fast storage and retrieve to any of the memory devices canbe enabled through the use of cache memory, that can be closelyassociated with one or more CPU 1041, GPU 1042, mass storage 1047, ROM1045, RAM 1046, and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture 1000, and specifically the core 1040 can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core 1040 that are of non-transitorynature, such as core-internal mass storage 1047 or ROM 1045. Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core 1040. A computer-readablemedium can include one or more memory devices or chips, according toparticular needs. The software can cause the core 1040 and specificallythe processors therein (including CPU, GPU, FPGA, and the like) toexecute particular processes or particular parts of particular processesdescribed herein, including defining data structures stored in RAM 1046and modifying such data structures according to the processes defined bythe software. In addition or as an alternative, the computer system canprovide functionality as a result of logic hardwired or otherwiseembodied in a circuit (for example: accelerator 1044), which can operatein place of or together with software to execute particular processes orparticular parts of particular processes described herein. Reference tosoftware can encompass logic, and vice versa, where appropriate.Reference to a computer-readable media can encompass a circuit (such asan integrated circuit (IC)) storing software for execution, a circuitembodying logic for execution, or both, where appropriate. The presentdisclosure encompasses any suitable combination of hardware andsoftware.

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method of decoding an encoded video bitstream using at least one processor, the method comprising: obtaining a first flag indicating whether a constant picture size is used in a coded video sequence including a current picture; based on the first flag indicating that the constant picture size is used, decoding the current picture without performing reference picture resampling; based on the first flag indicating that the constant picture size is not used, obtaining a second flag indicating whether a conformance window size is signaled; based on the second flag indicating that the conformance window size is signaled: obtaining the conformance window size, determining a resampling ratio between the current picture and a reference picture based on the conformance window size, and performing the reference picture resampling on the current picture using the resampling ratio.
 2. The method of claim 1, wherein the conformance window size is signaled as at least one offset distance from a border of the current picture.
 3. The method of claim 1, wherein the first flag is signaled in a sequence parameter set (SPS), and wherein the second flag is signaled in one from among the SPS and a picture parameter set (PPS).
 4. The method of claim 3, wherein the second flag is signaled in the SPS, and indicates whether SPS conformance window parameters are signaled in the SPS.
 5. The method of claim 4, wherein based on the second flag indicating that the SPS conformance window parameters are signaled in the SPS, the conformance window size is obtained based on the SPS conformance window parameters.
 6. The method of claim 4, further comprising, based on the first flag indicating that the picture size is not constant, obtaining a third flag indicating whether PPS conformance window parameters are signaled in the PPS.
 7. The method of claim 6, wherein based on the second flag indicating that the SPS conformance window parameters are signaled in the SPS, and the third flag indicating that the PPS conformance window parameters are not signaled in the PPS, the conformance window size is obtained based on the SPS conformance window parameters.
 8. The method of claim 6, wherein based on the second flag indicating that the SPS conformance window parameters are not signaled in the SPS, and the third flag indicating that the PPS conformance window parameters are signaled in the PPS, the conformance window size is obtained based on the PPS conformance window parameters.
 9. A device for decoding an encoded video bitstream, the device comprisin at least one memory configured to store program code; and at least one processor configured to read the program code and operate as instructed by the program code, the program code including: first obtaining code configured to cause the at least one processor to obtain a first flag indicating whether a constant picture size is used in a coded video sequence including a current picture; decoding code configured to cause the at least one processor to, based on the first g indicating that the constant picture size is used, decode the current picture without performing reference picture resampling; second obtaining code configured to cause the at least one processor o, based on the first flag indicating that the constant picture size is not used, obtain a second flag indicating whether a conformance window size is signaled; and performing code configured to cause the at least one processor to, based on the second flag indicating that the conformance window size is signaled, obtain the conformance window size, determine a resampling ratio between the current picture and a reference picture based on the conformance window size, and perform the reference picture resampling on the current picture using the resampling ratio.
 10. The device of claim
 9. wherein the conformance window size is signaled as at least one offset distance from a border of the current picture.
 11. The device of claim 9, wherein the firs a is signaled in a sequence parameter set (SPS), and wherein the second flag is signaled in one from among the SPS and a picture parameter set (PPS). 12, The device of claim 11, wherein the second flag is signaled in the SPS, and indicates whether SPS conformance window parameters are signaled in the SPS.
 13. The device of claim 12, wherein based on the second flag indicating that the SPS conformance window parameters are signaled in the SPS, the conformance window size is obtained based on the SPS conformance window parameters.
 14. The device of claim 12, wherein the program code further includes third obtaining code configured to cause the at least one processor to, based on the first flag indicating that the picture size is not constant, obtain a third flag indicating whether PPS conformance window parameters are signaled in the PPS.
 15. The device of claim 14, wherein based on the second flag indicating that the SPS conformance window parameters are signaled in the SPS, and the third flag indicating that the PPS conformance window parameters are not signaled in the PPS, the conformance window size is obtained based on the SPS conformance window parameters.
 16. The device of claim 14, wherein based on the second flag indicating that the SPS conformance window parameters are not signaled in the SPS, and the third flag indicating that the PPS conformance window parameters are signaled in the PPS, the conformance window size is obtained based on the PPS conformance window parameters.
 17. A non-transitory computer-readable medium storing instructions, the instructions comprising: one or more instructions that, when executed by one or more processors of a device for decoding an encoded video bitstream, cause the one or more processors to: obtain a first flag indicating whether a constant picture size is used in a coded video sequence including a current picture; based on the first flag indicating that the constant picture size is used, decode the current picture without performing reference picture resampling; based on the first flag indicating that the constant picture size is not used, obtain a second flag indicating whether a conformance window size is signaled; based on the second flag indicating that the conformance window size is signaled: obtain the conformance window size, determine a resampling ratio between the current picture and a reference picture based on the conformance window size, and perform the reference picture resampling on the current picture using the resampling ratio.
 18. The device of claim 17, wherein the first flag is signaled in a sequence parameter set (SPS), and wherein the second flag is signaled in one from among the SPS and a picture parameter set (PPS),
 19. The device of claim 18, wherein the second flag is signaled in the SPS, and indicates whether SPS conformance window parameters are signaled in the SPS.
 20. The device of claim 19, wherein the one or more instructions further cause the at least one processor to, based on the first flag indicating that the picture size is not constant, obtain a third flag indicating whether PPS conformance window parameters are signaled in the 